The various neceſſities of mankind induced alſo the judges very ſoon to depart from the rigour and ſimplicity of the rule's of the common law, and to allow a more minute and complex conſtruction upon conveyances to uſes than upon others. Hence it was adjudged, that the uſe need not always be executed the inſtant the conveyance is made: but, if it cannot take effect at that time, the operation of the ſtatute may wait till the uſe ſhall ariſe upon ſome future contingency, to happen within a reaſonable period of time; and in the mean while the antient uſe ſhall remain in the original grantor: as, when lands are conveyed to the uſe of A and B, after a marriage ſhall be had between them[1], or to the uſe of A and his heirs till B ſhall pay him a ſum of money, and then to the uſe of B and his heirs[2]. Which doctrine, when deviſes by will were again introduced, and conſidered as equivalent in point of conſtruction to declarations of uſes, was alſo adopted in favour of executory deviſes[3]. But herein theſe, which are called contingent or ſpringing, uſes differ from an executory deviſe; in that there muſt be a perſon ſeiſed to ſuch uſes at the time when the contingency happens, elſe they can never be executed by the ſtatute; and therefore, if the eſtate of the feoffee to ſuch uſe be deſtroyed by alienation or otherwiſe, before the contingency ariſes, the uſe is deſtroyed for ever[4]: whereas by an executory deviſe the freehold itſelf is transferred to the future deviſee. And, in both theſe caſes, a fee may be limited to take effect after a fee[5], becauſe, though that was forbidden by the common law in favour of the lord's eſcheat, yet, when the legal eſtate was not extended beyond one fee-ſimple, ſuch ſubſequent uſes (after a uſe in fee) were before the ſtatute permitted to be limited in equity; and then the ſtatute executed the legal eſtate in the ſame manner as the uſe before ſubſiſted. It was alſo held that a uſe, though executed, may change from one to another by circumſtances ex poſt facto[6]; as, if A makes a feoffment
